fluent

I need create boundary conditions for a face but this boundary conditions depend of a pressure. I have a pressure with a sin function. So I need: when pressure is negative la surface is a wall but when de pressure is positive la surface is an exit of gas an escape. Can you help me???? it's urgent

I'm not aware that Fluent can model this using its standard boundary conditions - you might need to code it as a UDF. Although I have experience at writing UDFs I would not know how to change boundary condition types while Fluent is running.